Optimization of desiccant usage in a mems package

ABSTRACT

A MEMS device may be package with a desiccant to provide a moisture-free environment. In order to avoid undesirable effects on the MEMS device, the desiccant may be selected or treated so as to be compatible with a particular MEMS device. This treatment may include baking of the desiccant to as to cause outgassing of moisture or other undesirable material. The structure of the MEMS device may also be altered to improve compatibility with particular desiccants.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to small scale electromechanical devices, such asmicroelectromechanical systems (MEMS) or nanoelectromechanical systems(NEMS) device.

2. Description of the Related Art

MEMS include micro mechanical elements, actuators, and electronics.Although the term MEMS is used through the specification forconvenience, it will be understood that the term is intended toencompass smaller-scale devices, such as NEMS. Micromechanical elementsmay be created using deposition, etching, and/or other micromachiningprocesses that etch away parts of substrates and/or deposited materiallayers or that add layers to form electrical and electromechanicaldevices. One type of MEMS device is called an interferometric modulator.As used herein, the term interferometric modulator or interferometriclight modulator refers to a device that selectively absorbs and/orreflects light using the principles of optical interference. In certainembodiments, an interferometric modulator may comprise a pair ofconductive plates, one or both of which may be transparent and/orreflective in whole or part and capable of relative motion uponapplication of an appropriate electrical signal. In a particularembodiment, one plate may comprise a stationary layer deposited on asubstrate and the other plate may comprise a metallic membrane separatedfrom the stationary layer by an air gap. As described herein in moredetail, the position of one plate in relation to another can change theoptical interference of light incident on the interferometric modulator.Such devices have a wide range of applications, and it would bebeneficial in the art to utilize and/or modify the characteristics ofthese types of devices so that their features can be exploited inimproving existing products and creating new products that have not yetbeen developed.

SUMMARY OF THE INVENTION

In one aspect, a method of encapsulating a MEMS device includesproviding a MEMS package, the MEMS package including a substratesupporting a MEMS device, a backplate sealed to the substrate to formthe MEMS package, and a desiccant within the MEMS package, the desiccantincluding calcium oxide and a polymer binding, and heating the MEMSpackage to cause moisture within the package to be chemically absorbedby the calcium oxide.

In another aspect, a method of encapsulating a MEMS device includesproviding a substrate supporting a MEMS device, providing a backplate,providing a desiccant, the desiccant including calcium oxide andpolytetrafluoroethylene, baking the desiccant under conditionssufficient to cause outgassing of organics retained therein, and sealingthe backplate to the substrate to form a MEMS package encapsulating theMEMS device and the desiccant.

In another aspect, a method of encapsulating a MEMS device includesproviding a substrate supporting a MEMS device, providing a backplate,providing a desiccant, the desiccant including calcium oxide and apolymer binding, baking the desiccant under conditions sufficient tocause outgassing of solvents retained therein, and sealing the backplateto the substrate to form a MEMS package encapsulating the MEMS deviceand the desiccant.

In another aspect, a method of encapsulating a MEMS device includesproviding a substrate supporting a MEMS device, the MEMS deviceincluding a movable layer, where the movable layer includes aluminum andneodymium, providing a backplate, providing a desiccant, and sealing thebackplate to the substrate to form a MEMS package encapsulating the MEMSdevice and the desiccant.

In another aspect, A MEMS package is provided, the MEMS packageincluding a substrate supporting a MEMS device including a movablelayer, where the movable layer includes aluminum and neodymium, abackplate sealed to the substrate to form a MEMS package encapsulatingthe MEMS device, and a desiccant sealed within the MEMS package.

In another aspect, a MEMS device package is provided, the MEMS devicepackage including means for absorbing moisture, first means forsupporting the absorbing means, second means for supporting a MEMSstructure, where the MEMS structure includes means for inhibiting theformation of protrusions on a surface of the MEMS structure, and meansfor sealing the second supporting means to the first supporting means.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view depicting a portion of one embodiment of aninterferometric modulator display in which a movable reflective layer ofa first interferometric modulator is in a relaxed position and a movablereflective layer of a second interferometric modulator is in an actuatedposition.

FIG. 2 is a system block diagram illustrating one embodiment of anelectronic device incorporating a 3×3 interferometric modulator display.

FIG. 3 is a diagram of movable mirror position versus applied voltagefor one exemplary embodiment of an interferometric modulator of FIG. 1.

FIG. 4 is an illustration of a set of row and column voltages that maybe used to drive an interferometric modulator display.

FIG. 5A illustrates one exemplary frame of display data in the 3×3interferometric modulator display of FIG. 2.

FIG. 5B illustrates one exemplary timing diagram for row and columnsignals that may be used to write the frame of FIG. 5A.

FIGS. 6A and 6B are system block diagrams illustrating an embodiment ofa visual display device comprising a plurality of interferometricmodulators.

FIG. 7A is a cross section of the device of FIG. 1.

FIG. 7B is a cross section of an alternative embodiment of aninterferometric modulator.

FIG. 7C is a cross section of another alternative embodiment of aninterferometric modulator.

FIG. 7D is a cross section of yet another alternative embodiment of aninterferometric modulator.

FIG. 7E is a cross section of an additional alternative embodiment of aninterferometric modulator.

FIG. 8 is a cross-section of an embodiment of a MEMS device packagecomprising a desiccant supported by a shaped backplate.

FIG. 9 is a cross-section of an embodiment of an exemplary MEMS devicecomprising specific layers and sublayers.

FIG. 10 is a process flow illustrating steps in the assembly of a MEMSpackage, wherein the desiccant is baked prior to assembly.

FIG. 11 is a process flow illustrating steps in the assembly of a MEMSpackage wherein the assembled MEMS package is baked.

DETAILED DESCRIPTION

The following detailed description is directed to certain specificembodiments of the invention. However, the invention can be embodied ina multitude of different ways. In this description, reference is made tothe drawings wherein like parts are designated with like numeralsthroughout. As will be apparent from the following description, theembodiments may be implemented in any device that is configured todisplay an image, whether in motion (e.g., video) or stationary (e.g.,still image), and whether textual or pictorial. More particularly, it iscontemplated that the embodiments may be implemented in or associatedwith a variety of electronic devices such as, but not limited to, mobiletelephones, wireless devices, personal data assistants (PDAs), hand-heldor portable computers, GPS receivers/navigators, cameras, MP3 players,camcorders, game consoles, wrist watches, clocks, calculators,television monitors, flat panel displays, computer monitors, autodisplays (e.g., odometer display, etc.), cockpit controls and/ordisplays, display of camera views (e.g., display of a rear view camerain a vehicle), electronic photographs, electronic billboards or signs,projectors, architectural structures, packaging, and aestheticstructures (e.g., display of images on a piece of jewelry). MEMS devicesof similar structure to those described herein can also be used innon-display applications such as in electronic switching devices.

In certain cases, it is desirable to package a MEMS device with adesiccant in order to maintain a substantially moisture-free environmentwithin a MEMS package to extend the lifetime of the MEMS device.However, particular combinations of desiccants and MEMS devices mayresult in undesirable effects, including damage to the MEMS devices. Incertain cases, such undesirable effects can be avoided through theselection of appropriate materials for use in the MEMS package. Certaindesiccants, such as those comprising calcium oxide in a polymer matrix,have been effective at minimizing or eliminating certain undesirableeffects, such as the wrinkling of a movable layer in an encapsulatedMEMS device. In addition, the use of particular materials in the MEMSdevice has been shown to be effective in controlling other undesirableeffects. The use of a movable layer which includes a mixture of aluminumand neodymium has been effective in preventing the growth of protrusionson the movable layer and on facing dielectric layers.

In other embodiments, the methods of encapsulating MEMS devices may bemodified to remove moisture or other materials which may interfere withthe operation of the MEMS device. In some cases, heating beyond thatwhich is necessary to cure desiccants or adhesives may be used to induceoutgassing of moisture or other undesirable material. This may be doneprior to encapsulation, such that the outgassed material is removed fromthe desiccant before the desiccant is encapsulated. This may also bedone after encapsulation, so as to drive moisture within the packageinto the active desiccant material. Outgassing of water can also beaccomplished through storage of the desiccant in a dry environment for aperiod of time, but the addition of heat or vacuum can significantlyreduce the necessary time.

the issues may be addressed through the use of desiccants found to beparticularly suitable for certain MEMS devices, as well as themodification of MEMS devices to avoid undesirable effects. In othercases, the desiccants may be treated in some manner, such as through theapplication of heat, in order to reduce the presence of potentiallyharmful components. In other cases, the assembly process may bemodified, such as by assembling the MEMS package in a moisture-freeenvironment.

One interferometric modulator display embodiment comprising aninterferometric MEMS display element is illustrated in FIG. 1. In thesedevices, the pixels are in either a bright or dark state. In the bright(“on” or “open”) state, the display element reflects a large portion ofincident visible light to a user. When in the dark (“off” or “closed”)state, the display element reflects little incident visible light to theuser. Depending on the embodiment, the light reflectance properties ofthe “on” and “off” states may be reversed. MEMS pixels can be configuredto reflect predominantly at selected colors, allowing for a colordisplay in addition to black and white.

FIG. 1 is an isometric view depicting two adjacent pixels in a series ofpixels of a visual display, wherein each pixel comprises a MEMSinterferometric modulator. In some embodiments, an interferometricmodulator display comprises a row/column array of these interferometricmodulators. Each interferometric modulator includes a pair of reflectivelayers positioned at a variable and controllable distance from eachother to form a resonant optical gap with at least one variabledimension. In one embodiment, one of the reflective layers may be movedbetween two positions. In the first position, referred to herein as therelaxed position, the movable reflective layer is positioned at arelatively large distance from a fixed partially reflective layer. Inthe second position, referred to herein as the actuated position, themovable reflective layer is positioned more closely adjacent to thepartially reflective layer. Incident light that reflects from the twolayers interferes constructively or destructively depending on theposition of the movable reflective layer, producing either an overallreflective or non-reflective state for each pixel.

The depicted portion of the pixel array in FIG. 1 includes two adjacentinterferometric modulators 12 a and 12 b. In the interferometricmodulator 12 a on the left, a movable reflective layer 14 a isillustrated in a relaxed position at a predetermined distance from anoptical stack 16 a, which includes a partially reflective layer. In theinterferometric modulator 12 b on the right, the movable reflectivelayer 14 b is illustrated in an actuated position adjacent to theoptical stack 16 b.

The optical stacks 16 a and 16 b (collectively referred to as opticalstack 16), as referenced herein, typically comprise several fusedlayers, which can include an electrode layer, such as indium tin oxide(ITO), a partially reflective layer, such as chromium, and a transparentdielectric. The optical stack 16 is thus electrically conductive,partially transparent, and partially reflective, and may be fabricated,for example, by depositing one or more of the above layers onto atransparent substrate 20. The partially reflective layer can be formedfrom a variety of materials that are partially reflective such asvarious metals, semiconductors, and dielectrics. The partiallyreflective layer can be formed of one or more layers of materials, andeach of the layers can be formed of a single material or a combinationof materials.

In some embodiments, the layers of the optical stack 16 are patternedinto parallel strips, and may form row electrodes in a display device asdescribed further below. The movable reflective layers 14 a, 14 b may beformed as a series of parallel strips of a deposited metal layer orlayers (orthogonal to the row electrodes of 16 a, 16 b) deposited on topof posts 18 and an intervening sacrificial material deposited betweenthe posts 18. When the sacrificial material is etched away, the movablereflective layers 14 a, 14 b are separated from the optical stacks 16 a,16 b by a defined gap 19. A highly conductive and reflective materialsuch as aluminum may be used for the reflective layers 14, and thesestrips may form column electrodes in a display device.

With no applied voltage, the gap 19 remains between the movablereflective layer 14 a and optical stack 16 a, with the movablereflective layer 14 a in a mechanically relaxed state, as illustrated bythe pixel 12 a in FIG. 1. However, when a potential difference isapplied to a selected row and column, the capacitor formed at theintersection of the row and column electrodes at the corresponding pixelbecomes charged, and electrostatic forces pull the electrodes together.If the voltage is high enough, the movable reflective layer 14 isdeformed and is forced against the optical stack 16. A dielectric layer(not illustrated in this Figure) within the optical stack 16 may preventshorting and control the separation distance between layers 14 and 16,as illustrated by pixel 12 b on the right in FIG. 1. The behavior is thesame regardless of the polarity of the applied potential difference. Inthis way, row/column actuation that can control the reflective vs.non-reflective pixel states is analogous in many ways to that used inconventional LCD and other display technologies.

FIGS. 2 through 5B illustrate one exemplary process and system for usingan array of interferometric modulators in a display application.

FIG. 2 is a system block diagram illustrating one embodiment of anelectronic device that may incorporate aspects of the invention. In theexemplary embodiment, the electronic device includes a processor 21which may be any general purpose single- or multi-chip microprocessorsuch as an ARM, Pentium®, Pentium II®, Pentium III®, Pentium IV®,Pentium® Pro, an 8051, a MIPS®, a Power PC®, an ALPHA®, or any specialpurpose microprocessor such as a digital signal processor,microcontroller, or a programmable gate array. As is conventional in theart, the processor 21 may be configured to execute one or more softwaremodules. In addition to executing an operating system, the processor maybe configured to execute one or more software applications, including aweb browser, a telephone application, an email program, or any othersoftware application.

In one embodiment, the processor 21 is also configured to communicatewith an array driver 22. In one embodiment, the array driver 22 includesa row driver circuit 24 and a column driver circuit 26 that providesignals to a display array or panel 30. The cross section of the arrayillustrated in FIG. 1 is shown by the lines 1-1 in FIG. 2. For MEMSinterferometric modulators, the row/column actuation protocol may takeadvantage of a hysteresis property of these devices illustrated in FIG.3. It may require, for example, a 10 volt potential difference to causea movable layer to deform from the relaxed state to the actuated state.However, when the voltage is reduced from that value, the movable layermaintains its state as the voltage drops back below 10 volts. In theexemplary embodiment of FIG. 3, the movable layer does not relaxcompletely until the voltage drops below 2 volts. Thus, there exists awindow of applied voltage, about 3 to 7 V in the example illustrated inFIG. 3, within which the device is stable in either the relaxed oractuated state. This is referred to herein as the “hysteresis window” or“stability window.” For a display array having the hysteresischaracteristics of FIG. 3, the row/column actuation protocol can bedesigned such that during row strobing, pixels in the strobed row thatare to be actuated are exposed to a voltage difference of about 10volts, and pixels that are to be relaxed are exposed to a voltagedifference of close to zero volts. After the strobe, the pixels areexposed to a steady state voltage difference of about 5 volts such thatthey remain in whatever state the row strobe put them in. After beingwritten, each pixel sees a potential difference within the “stabilitywindow” of 3-7 volts in this example. This feature makes the pixeldesign illustrated in FIG. 1 stable under the same applied voltageconditions in either an actuated or relaxed pre-existing state. Sinceeach pixel of the interferometric modulator, whether in the actuated orrelaxed state, is essentially a capacitor formed by the fixed and movingreflective layers, this stable state can be held at a voltage within thehysteresis window with almost no power dissipation. Essentially nocurrent flows into the pixel if the applied potential is fixed.

In typical applications, a display frame may be created by asserting theset of column electrodes in accordance with the desired set of actuatedpixels in the first row. A row pulse is then applied to the row 1electrode, actuating the pixels corresponding to the asserted columnlines. The asserted set of column electrodes is then changed tocorrespond to the desired set of actuated pixels in the second row. Apulse is then applied to the row 2 electrode, actuating the appropriatepixels in row 2 in accordance with the asserted column electrodes. Therow 1 pixels are unaffected by the row 2 pulse, and remain in the statethey were set to during the row 1 pulse. This may be repeated for theentire series of rows in a sequential fashion to produce the frame.Generally, the frames are refreshed and/or updated with new display databy continually repeating this process at some desired number of framesper second. A wide variety of protocols for driving row and columnelectrodes of pixel arrays to produce display frames are also well knownand may be used in conjunction with the present invention.

FIGS. 4, 5A, and 5B illustrate one possible actuation protocol forcreating a display frame on the 3×3 array of FIG. 2. FIG. 4 illustratesa possible set of column and row voltage levels that may be used forpixels exhibiting the hysteresis curves of FIG. 3. In the FIG. 4embodiment, actuating a pixel involves setting the appropriate column to−V_(bias), and the appropriate row to +ΔV, which may correspond to −5volts and +5 volts, respectively. Relaxing the pixel is accomplished bysetting the appropriate column to +V_(bias), and the appropriate row tothe same +ΔV, producing a zero volt potential difference across thepixel. In those rows where the row voltage is held at zero volts, thepixels are stable in whatever state they were originally in, regardlessof whether the column is at +V_(bias), or −V_(bias). As is alsoillustrated in FIG. 4, it will be appreciated that voltages of oppositepolarity than those described above can be used, e.g., actuating a pixelcan involve setting the appropriate column to +V_(bias), and theappropriate row to −ΔV. In this embodiment, releasing the pixel isaccomplished by setting the appropriate column to −V_(bias), and theappropriate row to the same −ΔV, producing a zero volt potentialdifference across the pixel.

FIG. 5B is a timing diagram showing a series of row and column signalsapplied to the 3×3 array of FIG. 2 which will result in the displayarrangement illustrated in FIG. 5A, where actuated pixels arenon-reflective. Prior to writing the frame illustrated in FIG. 5A, thepixels can be in any state, and in this example, all the rows are at 0volts, and all the columns are at +5 volts. With these applied voltages,all pixels are stable in their existing actuated or relaxed states.

In the FIG. 5A frame, pixels (1,1), (1,2), (2,2), (3,2) and (3,3) areactuated. To accomplish this, during a “line time” for row 1, columns 1and 2 are set to −5 volts, and column 3 is set to +5 volts. This doesnot change the state of any pixels, because all the pixels remain in the3-7 volt stability window. Row 1 is then strobed with a pulse that goesfrom 0, up to 5 volts, and back to zero. This actuates the (1,1) and(1,2) pixels and relaxes the (1,3) pixel. No other pixels in the arrayare affected. To set row 2 as desired, column 2 is set to −5 volts, andcolumns 1 and 3 are set to +5 volts. The same strobe applied to row 2will then actuate pixel (2,2) and relax pixels (2,1) and (2,3). Again,no other pixels of the array are affected. Row 3 is similarly set bysetting columns 2 and 3 to −5 volts, and column 1 to +5 volts. The row 3strobe sets the row 3 pixels as shown in FIG. 5A. After writing theframe, the row potentials are zero, and the column potentials can remainat either +5 or −5 volts, and the display is then stable in thearrangement of FIG. 5A. It will be appreciated that the same procedurecan be employed for arrays of dozens or hundreds of rows and columns. Itwill also be appreciated that the timing, sequence, and levels ofvoltages used to perform row and column actuation can be varied widelywithin the general principles outlined above, and the above example isexemplary only, and any actuation voltage method can be used with thesystems and methods described herein.

FIGS. 6A and 6B are system block diagrams illustrating an embodiment ofa display device 40. The display device 40 can be, for example, acellular or mobile telephone. However, the same components of displaydevice 40 or slight variations thereof are also illustrative of varioustypes of display devices such as televisions and portable media players.

The display device 40 includes a housing 41, a display 30, an antenna43, a speaker 45, an input device 48, and a microphone 46. The housing41 is generally formed from any of a variety of manufacturing processesas are well known to those of skill in the art, including injectionmolding and vacuum forming. In addition, the housing 41 may be made fromany of a variety of materials, including, but not limited to, plastic,metal, glass, rubber, and ceramic, or a combination thereof. In oneembodiment, the housing 41 includes removable portions (not shown) thatmay be interchanged with other removable portions of different color, orcontaining different logos, pictures, or symbols.

The display 30 of exemplary display device 40 may be any of a variety ofdisplays, including a bi-stable display, as described herein. In otherembodiments, the display 30 includes a flat-panel display, such asplasma, EL, OLED, STN LCD, or TFT LCD as described above, or anon-flat-panel display, such as a CRT or other tube device, as is wellknown to those of skill in the art. However, for purposes of describingthe present embodiment, the display 30 includes an interferometricmodulator display, as described herein.

The components of one embodiment of exemplary display device 40 areschematically illustrated in FIG. 6B. The illustrated exemplary displaydevice 40 includes a housing 41 and can include additional components atleast partially enclosed therein. For example, in one embodiment, theexemplary display device 40 includes a network interface 27 thatincludes an antenna 43, which is coupled to a transceiver 47. Thetransceiver 47 is connected to a processor 21, which is connected toconditioning hardware 52. The conditioning hardware 52 may be configuredto condition a signal (e.g., filter a signal). The conditioning hardware52 is connected to a speaker 45 and a microphone 46. The processor 21 isalso connected to an input device 48 and a driver controller 29. Thedriver controller 29 is coupled to a frame buffer 28 and to an arraydriver 22, which in turn is coupled to a display array 30. A powersupply 50 provides power to all components as required by the particularexemplary display device 40 design.

The network interface 27 includes the antenna 43 and the transceiver 47so that the exemplary display device 40 can communicate with one or moredevices over a network. In one embodiment, the network interface 27 mayalso have some processing capabilities to relieve requirements of theprocessor 21. The antenna 43 is any antenna known to those of skill inthe art for transmitting and receiving signals. In one embodiment, theantenna transmits and receives RF signals according to the IEEE 802.11standard, including IEEE 802.11(a), (b), or (g). In another embodiment,the antenna transmits and receives RF signals according to the BLUETOOTHstandard. In the case of a cellular telephone, the antenna is designedto receive CDMA, GSM, AMPS, or other known signals that are used tocommunicate within a wireless cell phone network. The transceiver 47pre-processes the signals received from the antenna 43 so that they maybe received by and further manipulated by the processor 21. Thetransceiver 47 also processes signals received from the processor 21 sothat they may be transmitted from the exemplary display device 40 viathe antenna 43.

In an alternative embodiment, the transceiver 47 can be replaced by areceiver. In yet another alternative embodiment, network interface 27can be replaced by an image source, which can store or generate imagedata to be sent to the processor 21. For example, the image source canbe a digital video disc (DVD) or a hard-disc drive that contains imagedata, or a software module that generates image data.

Processor 21 generally controls the overall operation of the exemplarydisplay device 40. The processor 21 receives data, such as compressedimage data from the network interface 27 or an image source, andprocesses the data into raw image data or into a format that is readilyprocessed into raw image data. The processor 21 then sends the processeddata to the driver controller 29 or to frame buffer 28 for storage. Rawdata typically refers to the information that identifies the imagecharacteristics at each location within an image. For example, suchimage characteristics can include color, saturation, and gray-scalelevel.

In one embodiment, the processor 21 includes a microcontroller, CPU, orlogic unit to control operation of the exemplary display device 40.Conditioning hardware 52 generally includes amplifiers and filters fortransmitting signals to the speaker 45, and for receiving signals fromthe microphone 46. Conditioning hardware 52 may be discrete componentswithin the exemplary display device 40, or may be incorporated withinthe processor 21 or other components.

The driver controller 29 takes the raw image data generated by theprocessor 21 either directly from the processor 21 or from the framebuffer 28 and reformats the raw image data appropriately for high speedtransmission to the array driver 22. Specifically, the driver controller29 reformats the raw image data into a data flow having a raster-likeformat, such that it has a time order suitable for scanning across thedisplay array 30. Then the driver controller 29 sends the formattedinformation to the array driver 22. Although a driver controller 29,such as a LCD controller, is often associated with the system processor21 as a stand-alone Integrated Circuit (IC), such controllers may beimplemented in many ways. They may be embedded in the processor 21 ashardware, embedded in the processor 21 as software, or fully integratedin hardware with the array driver 22.

Typically, the array driver 22 receives the formatted information fromthe driver controller 29 and reformats the video data into a parallelset of waveforms that are applied many times per second to the hundredsand sometimes thousands of leads coming from the display's x-y matrix ofpixels.

In one embodiment, the driver controller 29, array driver 22, anddisplay array 30 are appropriate for any of the types of displaysdescribed herein. For example, in one embodiment, driver controller 29is a conventional display controller or a bi-stable display controller(e.g., an interferometric modulator controller). In another embodiment,array driver 22 is a conventional driver or a bi-stable display driver(e.g., an interferometric modulator display). In one embodiment, adriver controller 29 is integrated with the array driver 22. Such anembodiment is common in highly integrated systems such as cellularphones, watches, and other small area displays. In yet anotherembodiment, display array 30 is a typical display array or a bi-stabledisplay array (e.g., a display including an array of interferometricmodulators).

The input device 48 allows a user to control the operation of theexemplary display device 40. In one embodiment, input device 48 includesa keypad, such as a QWERTY keyboard or a telephone keypad, a button, aswitch, a touch-sensitive screen, or a pressure- or heat-sensitivemembrane. In one embodiment, the microphone 46 is an input device forthe exemplary display device 40. When the microphone 46 is used to inputdata to the device, voice commands may be provided by a user forcontrolling operations of the exemplary display device 40.

Power supply 50 can include a variety of energy storage devices as arewell known in the art. For example, in one embodiment, power supply 50is a rechargeable battery, such as a nickel-cadmium battery or a lithiumion battery. In another embodiment, power supply 50 is a renewableenergy source, a capacitor, or a solar cell including a plastic solarcell, and solar-cell paint. In another embodiment, power supply 50 isconfigured to receive power from a wall outlet.

In some embodiments, control programmability resides, as describedabove, in a driver controller which can be located in several places inthe electronic display system. In some embodiments, controlprogrammability resides in the array driver 22. Those of skill in theart will recognize that the above-described optimizations may beimplemented in any number of hardware and/or software components and invarious configurations.

The details of the structure of interferometric modulators that operatein accordance with the principles set forth above may vary widely. Forexample, FIGS. 7A-7E illustrate five different embodiments of themovable reflective layer 14 and its supporting structures. FIG. 7A is across section of the embodiment of FIG. 1, where a strip of metalmaterial 14 is deposited on orthogonally extending supports 18. In FIG.7B, the moveable reflective layer 14 is attached to supports at thecorners only, on tethers 32. In FIG. 7C, the moveable reflective layer14 is suspended from a deformable layer 34, which may comprise aflexible metal. The deformable layer 34 connects, directly orindirectly, to the substrate 20 around the perimeter of the deformablelayer 34. These connections are herein referred to as support posts. Theembodiment illustrated in FIG. 7D has support post plugs 42 upon whichthe deformable layer 34 rests. The movable reflective layer 14 remainssuspended over the gap, as in FIGS. 7A-7C, but the deformable layer 34does not form the support posts by filling holes between the deformablelayer 34 and the optical stack 16. Rather, the support posts are formedof a planarization material, which is used to form support post plugs42. The embodiment illustrated in FIG. 7E is based on the embodimentshown in FIG. 7D, but may also be adapted to work with any of theembodiments illustrated in FIGS. 7A-7C, as well as additionalembodiments not shown. In the embodiment shown in FIG. 7E, an extralayer of metal or other conductive material has been used to form a busstructure 44. This allows signal routing along the back of theinterferometric modulators, eliminating a number of electrodes that mayotherwise have had to be formed on the substrate 20.

In embodiments such as those shown in FIG. 7, the interferometricmodulators function as direct-view devices, in which images are viewedfrom the front side of the transparent substrate 20, the side oppositeto that upon which the modulator is arranged. In these embodiments, thereflective layer 14 optically shields the portions of theinterferometric modulator on the side of the reflective layer oppositethe substrate 20, including the deformable layer 34. This allows theshielded areas to be configured and operated upon without negativelyaffecting the image quality. Such shielding allows the bus structure 44in FIG. 7E, which provides the ability to separate the opticalproperties of the modulator from the electromechanical properties of themodulator, such as addressing and the movements that result from thataddressing. This separable modulator architecture allows the structuraldesign and materials used for the electromechanical aspects and theoptical aspects of the modulator to be selected and to functionindependently of each other. Moreover, the embodiments shown in FIGS.7C-7E have additional benefits deriving from the decoupling of theoptical properties of the reflective layer 14 from its mechanicalproperties, which are carried out by the deformable layer 34. Thisallows the structural design and materials used for the reflective layer14 to be optimized with respect to the optical properties, and thestructural design and materials used for the deformable layer 34 to beoptimized with respect to desired mechanical properties.

Interferometric modulators, and other MEMS devices—particularly thosehaving large surfaces which come into contact with one another—areparticularly sensitive to failure due to humidity and moisture buildup.In order to protect such MEMS devices from humidity, and other forms ofenvironmental or mechanical interference, such MEMS devices are oftensealed within a protective package. In many embodiments, a substratecomprising an array of MEMS devices is sealed to a back plate in orderto provide such a package. Although the below embodiments are primarilydiscussed with respect to interferometric modulators, it will beunderstood that a wide variety of MEMS devices may benefit from theprotection afforded by such a package, and can be used in conjunctionwith the structures and processes described herein.

FIG. 8 depicts an embodiment of a package 100, which may, for example,form a part of a display device. The package 100 comprises alight-transmissive substrate 110, which may preferably be asubstantially transparent substrate, through which a viewer may view anarray 120 of interferometric modulators. The substrate 100 thus providesone means for supporting the array of interferometric modulators, orother MEMS structure. The light-transmissive substrate 110 is sealed toa backplate 130 via seal 140, providing a cavity 150 in which theinterferometric modulator array 120 resides. Also within the cavity 150is a layer of desiccant 160, which in the illustrated embodiment ispositioned within a recess 170 in the backplate 130. The desiccant 160thus provides one means for absorbing moistures, the backplate 130provides one means for supporting the desiccant, and the seal 140provides one means for sealing the backplate 130 to the substrate 100.Because the backplate 130 is a shaped backplate comprising recess 170,the height (a) of the seal 140 can be advantageously minimized whilestill providing sufficient clearance for the desiccant 160 to bepositioned without substantial risk of mechanical interference with theinterferometric modulator array 120. It will be understood, however,that the desiccant 160 may be placed at any of a variety of locations inthe package 100, and may be placed at multiple locations throughout thepackage 100.

A wide variety of materials can be used to form the backplate 130,including glass, metal, foil, polymer, plastic, ceramic, orsemiconductor materials (e.g. silicon). The substrate 110 may comprise,for example, glass, plastic, or transparent polymer. The seal 140 may bea non-hermetic seal, and comprise a material such as a conventionalepoxy-based adhesive. In other embodiments, the seal 140 may be apolyisobutylene (sometimes called butyl rubber and other times PIB),o-rings, polyurethane, thin film metal weld, liquid spin-on glass,solder, polymers, or plastics, among other types of seals that may havea range of permeability of water vapor of about 0.2-4.7 g mm/m² kPa day.In other embodiments, the seal 140 may be a hermetic seal.

MEMS devices, and in particular MEMS devices such as interferometricmodulators, are sensitive to environmental conditions such as humidity.Generally, it is desirable to minimize the permeation of water vaporinto the package structure and thus control the environment inside thepackage 100 and hermetically seal it to ensure that the environmentremains constant. When the humidity within the package 100 exceeds alevel beyond which surface tension from the moisture becomes higher thanthe restoration force of a movable element (e.g., the movable mirrors 14a, 14 b described above with respect to FIG. 1) in a MEMS device such asthe interferometric modulator array 120, the movable element may becomestuck to an adjacent surface for a prolonged period of time, and maybecome permanently stuck. Because of the large contact areas andcomparatively low restoration forces of the movable mirrors in aninterferometric modulator, interferometric modulators are particularlysusceptible to failure due to permanent adhesion. This permanentadhesion may be brought about by high humidity levels or other outgassedmaterials which may collect on a surface of a MEMS device and result instiction between a movable layer and an adjacent layer. Humidity withinthe package 100 can contribute to other undesirable effects, such as thedevelopment of discoloration, which is particularly undesirable in anoptical device such as an interferometric modulator display.

A desiccant such as desiccant 160 may be used to control moistureresident within the package 100. Desiccants may be used for packagesthat have either hermetic or non-hermetic seals. In packages having ahermetic seal, desiccants are typically used to control moistureresident within the interior of the package. In packages having anon-hermetic seal, a desiccant may be used to control moisture movinginto the package from the environment. The skilled artisan willappreciate that a desiccant may not be necessary for a hermeticallysealed package, but may be desirable to control moisture resident withinthe package or to capture outgases materials or materials from surfacesinside the package.

According to the embodiments described herein, the desiccant preferablyis configured to absorb water molecules that permeate the displaypackage structure once it has been manufactured as well as aftersealing. As can be appreciated, the desiccant maintains a low humidityenvironment within the package and prevents water vapor from adverselyaffecting the operation of the MEMS devices and any associated displayelectronics.

Generally, any substance that can trap moisture while not interferingwith the optical properties of the interferometric modulator array maybe used as the desiccant material 160. Suitable desiccant materialsinclude, but are not limited to, zeolites, calcium sulfate, calciumoxide, silica gel, molecular sieves, surface adsorbents, bulkadsorbents, and chemical reactants. Other desiccant materials includeindicating silica gel, which is silica gel with some of its granulescoated with cobalt chloride. The silica changes color as it becomessaturated with water. Calcium oxide is a material that relatively slowlyabsorbs water.

The desiccant may be in different forms, shapes, and sizes. In additionto being in solid or gel form, the desiccant material may alternately bein powder form. These powders may be inserted into a water vaporpermeable pouch, or directly into the package without a pouch, or may bemixed with an adhesive for application. In an alternative embodiment,the desiccant may be formed into different shapes, such as cylinders orsheets, before being applied inside the package. It should be realizedthat the desiccant 160 may take any form, and can be of any thicknessthat provides the proper desiccating function for the package 100.

The desiccant 160 may be applied within the package in a variety ofother ways, as well. In one embodiment, the desiccant 160 may bedeposited as part of the interferometric modulator array 120. In anotherembodiment, the desiccant material is applied inside the package as aspray or a drip coat. The desiccant may also be printed or sprayed ontoa surface of the interior of the package, or may be brushed on. Theportions of the backplate which are not intended to be covered bydesiccant may be protected by a mask layer. In another embodiment thedesiccant may also be embedded within the seal adhesive.

Typically, in packages containing desiccants, the lifetime expectationof the device may depend on the lifetime of the desiccant. When thedesiccant is fully consumed, the interferometric modulator array 120 mayfail to operate as sufficient moisture enters the cavity 150 and causesdamage to the array 120. The theoretical maximum lifetime of the displaydevice is determined by the water vapor flux into the cavity 150 as wellas the amount and type of desiccant material.

The theoretical lifetime of the device may be calculated with thefollowing equations:

${lifetime} = \frac{{desiccant\_ capacity}(g)}{{water\_ vapor}{\_ flux}\left( {g\text{/}{area}\text{/}{day}} \right)*{perimeter\_ seal}{\_ area}}$${{water}\mspace{14mu} {vapor}\mspace{14mu} {flux}} = {{- P}\frac{p}{t}}$

where P is the water vapor permeation coefficient for the perimeter seal280 and

$\frac{p}{t}$

is the water vapor pressure gradient across the width of the seal 280.

In the embodiment of a display having a hermetic seal, the lifetime ofthe device is not as dependent on the desiccant capacity, or thegeometry of the seal. In display devices wherein the seal 140 is nothermetic, the lifetime of the device is more dependent on the capacityof the desiccant to absorb and retain moisture.

In one embodiment, a method for assembling the package 100 includesfabricating the interferometric modulator array 120 on thelight-transmissive substrate 110. In certain embodiments, the backplate130 may be shaped via a sandblasting or etching process in order to formrecess 170. In other embodiments, the backplate 130 may be deformed toform a recess 170, or a pre-shaped backplate provided. The desiccant 160may then be applied in the recess 170, or elsewhere in the package. Theseal 140 is then put into place, and the backplate 130 and the lighttransmissive substrate 110 may be brought together to form the cavity150 which encapsulates both the desiccant 160 and the interferometricmodulator array 120.

In addition to providing a controlled package environment within thepackage with respect to humidity, the desiccant and/or the MEMSstructure may be selected to avoid or minimize the harmful effect ofother material within the package environment. Such harmful materialwithin the package environment may in certain embodiments be eliminatedor minimized. In other embodiments, the MEMS structure itself may bedesigned so as to minimize the effects of said potentially harmfulmaterial on the MEMS structure, potentially facilitating the use of awider range of desiccant materials.

Because of the wide range of potential interactions between the MEMSstructure and other components within and comprising the encapsulatingMEMS package, empirical studies of MEMS packages have provided insightregarding potential sources of damage to MEMS devices and steps whichmay be taken to prevent such damage.

Certain MEMS devices, such as interferometric modulators discussedabove, may comprise a movable mechanical layer. The mechanical layer canbe actuated as discussed above, and is returned to its original positiondue to tensile stress within the membrane. In particular embodiments,such a mechanical layer may include a layer of aluminum, which is highlyreflective, positioned adjacent an interferometric cavity, and a layerof nickel supporting the aluminum layer. FIG. 9 illustrates anembodiment of such a MEMS device 200. As can be seen, the MEMS devicecomprises a movable layer 210, also referred to as a mechanical layer,which includes an upper sublayer 212 and a lower sublayer 214. Incertain embodiments, the upper sublayer may comprise a material havingdesired mechanical properties, such as nickel, and the lower sublayer214 may comprise a highly reflective material, such as aluminum. Themovable layer 210 is spaced apart from the substrate 202 and layersdisposed thereon by supports 220 a and 220 b, which in the illustratedembodiment take the form of support posts. It will be understood that inother embodiments a wide variety of supports may be used, and that theportions movable layer may itself form a support. In an embodiment inwhich the MEMS device comprises an interferometric modulator, an opticalstack 230 may be formed on or adjacent the substrate 202. The opticalstack may comprise one or more of the following sublayers: a partiallyreflective layer 232, which may also be referred to as an absorber; aconductive layer 234, which may comprise a light-transmissive electrodeand may be formed from ITO; and an insulator layer 236, which maycomprise a light-transmissive dielectric layer.

Arrays of such MEMS devices packaged with certain desiccants, includingDynic HD-S desiccant, which is a fast acting desiccant in apolytetrafluoroethylene (Teflon) matrix, have exhibited damage to themechanical membrane over time. This damage has taken the form ofwrinkling of the mechanical membrane, along with a conversion fromtensile stress to compressive stress. This change in the stress withinthe mechanical layer can impair the functioning of the MEMS device, andis undesirable.

In one embodiment, the invention includes a method of fabricating a MEMSdevice including such a desiccant by providing additional heating to thedesiccant beyond what is necessary to cure the desiccant. This method iseffective to inhibit such wrinkling in an encapsulated device, whereinthe device is encapsulated with a fast-acting desiccant. The lifetime ofthe MEMS device encapsulated with the treated fast-acting desiccant maybe significantly extended. The fast-acting desiccant may be maintainedin a moisture-free environment, such as an N₂ atmosphere. In certainembodiments, the fast-acting desiccant is handled and treated in a glovebox.

By exposing the desiccant to an elevated temperature for an extendedperiod of time, outgassing of material such the potentially harmfulorganics is accelerated. Because this heating process may be performedprior to encapsulation of the desiccant, the outgassed material isremoved and does not affect the functioning of the MEMS device to beencapsulated. The rate of outgassing is at least partially dependent onthe temperature, as increased temperatures may lead to increased ratesof outgassing. It will be understood that a desired duration of theheating will be dependent on the temperature at which the desiccant isbaked. The desiccant alone can be baked at comparatively hightemperatures. In a particular embodiment, the desiccant alone is bakedat 170° C. for a period of as long as 24 hours, and in certainembodiments as long as 48 hours or longer.

Once the desiccant has been applied to a component of the MEMS package,such as the backplate, the use of high temperatures may be constrained.In particular, the adhesive securing the desiccant in place may bedamaged by baking above certain temperatures, and may be the limitingfactor in the maximum suitable baking temperature. The maximumtemperature to which a typical pressure-sensitive adhesive (PSA) shouldbe exposed without causing deterioration is about 120° C. In someembodiments, the baking temperature may be selected to be as high aspossible without causing such damage. Thus, the bakeout temperature maybe selected to be slightly below that temperature, thus, a typicalbaking temperature may be roughly 100° C. to 105° C., althoughtemperatures which are higher or lower may be used. The higher thetemperature, the faster the rate of outgassing may be. In furtherembodiments, a PSA selected to have a high temperature tolerance may beused. For example, some PSAs may be tolerant of temperatures up to orgreater than 175° C.

The necessary duration for the baking process will depend on factorssuch as the environment and the baking temperature, but in someembodiments, a baking time of one hour may be sufficient to outgas alarge amount or material. In an embodiment in which the desiccant isbaked prior to placement of the desiccant on the backplate, the processis an offline process which does not slow down the assembly line. Thus,significantly larger baking durations may be used, in order to provideadditional outgassing. In such an embodiment, the desiccant may be bakedfor as long as 24 to 48 hours, or longer.

The use of a vacuum or other dry environment can be used to accelerateoutgassing, as well, in order to permit equivalent outgassing at lowertemperatures or shorter durations. For example, a process which uses a105° C. back for 48 hours may be performed instead in a vacuum for 10hours at 90° C.

FIG. 10 illustrates an exemplary process flow 300 for baking a desiccantsuch as the calcium oxide desiccant in a polytetrafluoroethylenebinding. In step 302, the desiccant is placed within a controlledenvironment. In step 304, the desiccant may be adhered to a backplatevia an adhesive. It will be understood, however, as discussed above,that outgassing may be induced in other embodiments prior to adheringthe desiccant to a backplate, and that the step of adhering a desiccantto a backplate may occur at a later stage in the manufacturing process.

In step 306, the desiccant-bearing backplate is baked. As noted above,this baking process may be done via a hot plate or other heatingequipment. In embodiments in which the desiccant is adhered to abackplate, the temperature may be dependent upon the properties of theadhesive or of other components, rather than the desiccant. The durationmay be dependent upon the temperature at which the desiccant is beingbaked. In step 308, the desiccant-bearing backplate is used toencapsulate a MEMS device within a package.

As noted above, in certain embodiments, the desiccant may be baked in anN₂ atmosphere or other moisture-free environment. In certainembodiments, the desiccant may be baked in a glove box. The desiccantmay be baked by placing a backplate comprising the adhered desiccant ona heat plate within the controlled environment. Other method of heatingthe desiccant may also be used, and the desiccant need not be adhered toa backplate in order to be baked.

Another embodiment of the invention is a MEMS device having analuminum/nickel movable layer and a calcium oxide desiccant in a polymermatrix. A suitable polymer binding is a phenol binding, such as adesiccant produced by Cookson Electronics. When a desiccant comprisingcalcium oxide in a polymer binding is utilized in an encapsulated MEMSpackage, wrinkling of the mechanical layer does not present itself, evenafter long periods of time at elevated temperatures. Thus, in aparticular embodiment, a calcium oxide desiccant is used in a MEMSpackage comprising a MEMS device having an aluminum/nickel movablelayer, in order to inhibit wrinkling of the movable layer and aconversion from tensile to compressive stress. In a particularembodiment, the polymer binding comprises phenol. In other embodiments,the matrix may comprise other polymers, such as epoxies.

The use of a desiccant, such as a calcium oxide desiccant in a polymerbinding, can be optimized to inhibit alternate undesirable effects, aswell. One such undesirable effect is the development of protrusionswhich may develop, for instance, on the aluminum bottom surface of analuminum/nickel mechanical layer and on the surface of an dielectriclayer, such as an Al₂O₃ layer, which may be the upper layer 236 in anoptical stack 230 (see FIG. 9). It will be understood that in an opticaldevice such as an interferometric modulator, the development ofprotrusions on one of the two surfaces facing the interferometric cavitymay inhibit full actuation of the movable layer. As the movable layer isunable to contact the optical stack in the area adjacent such aprotrusion, that portion of the interferometric modulator may appear asa different color. The color shift can be due, for example, to thedistance between the partially reflective layer in the optical stack andthe reflective lower surface of the mechanical layer, as the distancewill be larger in the area surrounding the protrusion. In an embodimentin which the down state of an interferometric modulator corresponds to adark state, the area around the protrusion may appear white.

Referring again to FIG. 9, in certain embodiments, a MEMS device may beprovided in which the lower sublayer 214 of the movable layer 210comprises a mixture of aluminum and neodymium. It has been observed thatthe inclusion of neodymium in the aluminum sublayer of the movable layermay serve to control migration of aluminum and prevent the formation ofsuch protrusions on both the movable layer and the optical stack. Thus,in one embodiment, a MEMS package comprises a desiccant and a MEMSdevice comprising a movable layer, wherein the movable layer comprisesan aluminum and neodymium mixture. In certain embodiments, thepercentage of neodymium in the aluminum/neodymium layer is roughly 3%,and such an amount has been shown to be effective in controlling thegrowth of the protrusions, but in other embodiments, more or lessneodymium may be utilized. The layer containing neodymium thus providesone means for inhibiting formations of protrusions on a surface of theMEMS structure.

As discussed above, pre-baking of the desiccant may be useful in anembodiment in which a fast acting desiccant is used. However, pre-bakingof a calcium oxide and polymer desiccant is also useful. In certainembodiments, such a desiccant was provided in paste form, and wasapplied to a backplate and then cured to solidify the desiccant bydriving off the solvents that are used as the paste carrier solution.Recommended parameters for performing this curing bake were provided bythe vendor, which provide sufficient heating to solidify the pastedesiccant. However, it has been discovered that baking of such desiccantat temperatures well above the temperatures required to cure thedesiccant, and for periods of time longer than required to cure thedesiccant, has a beneficial effect on the operation of the device.

In particular, a desiccant comprising a polymer binding, may comprisesolvents which will outgas after encapsulation and interfere with theoperation of the device. In particular, the solvents may cause stictionbetween a movable layer and a layer which the movable layer contacts,inhibiting the release of the movable layer. By providing additionalbaking to the desiccant, the concentrations of these solvents weredrastically reduced. This was verified by Gas Chromatography MassSpectroscopy. In certain embodiments, the desiccant may be baked at atemperature which was as high as possible without degrading the polymerbinding itself. In particular embodiments, the desiccant may be baked attemperatures as high as approximately twice the recommended curingtemperature, and for a duration which is as long as or longer than sixtimes the recommended cure time. In a particular embodiment, such adesiccant was baked for three hours at a temperature of 250° C.,although it will be understood that different temperatures and durationsmay be used. For example, in certain embodiments, temperatures higherthan 220° C. may be used, and in further embodiments temperatures higherthan 280° C. may be used.

In addition to the removal of solvents during the elevated temperaturesof such a solvent bake, other additives included in the desiccant mayalso outgas, and may interfere with the operation of a MEMS device. Theoutgassing of other additives has been verified via ThermogravinometricAnalysis. In certain embodiments, the outgassing of such additives isprevented through the use of desiccants which do not contain suchadditives. In a particular embodiment, a desiccant comprises onlycalcium oxide, a polymer, and other materials needed to provide thedesired consistency (e.g., a paste).

As noted above, it may be desirable to package a fast-acting desiccantsuch as Dynic in a moisture-free environment. However, it may also bedesirable to package a slow-acting desiccant in a moisture-freeenvironment, as well. A slow-acting desiccant will generally not beconsumed during an ambient packaging process, and such a desiccant willstill have a significant useful lifetime after packaging. However,testing has shown that the polymer in a calcium oxide and polymerdesiccant will quickly absorb and retain moisture from an ambientenvironment. In terms of moisture absorption, the polymer may reachequilibrium with the environment in roughly 15 minutes, although thistime may vary based on the polymer. However, a potentially harmfulamount of water may be absorbed in as little as five minutes, or sooner.The polymer can typically absorb ˜1% of it dry weigh in water. Thisamount of water is significant when put into a small MEMS package suchas a display package and may raise the level of humidity in the packageto unacceptably high levels which may be as high or higher than 10,000ppm.

Once packaged, the water retained within the polymer will either beoutgassed or taken in by the calcium oxide in the desiccant. It waspreviously assumed that the water retained within the polymer would beabsorbed by the desiccant and would not interfere with the operation ofthe device. Unexpectedly, however, it has been shown that in aslow-acting desiccant, the water outgassed may damage or otherwiseinterfere with the operation of the MEMS device before the water isultimately absorbed by the desiccant. In particular, the water outgassedinto the package environment may cause stiction in a MEMS device havinga movable layer. Left alone, it may take several days for the water fromthe polymer to be absorbed by the desiccant.

Thus, in certain environments, a MEMS package comprising a slow-actingdesiccant can be assembled in a moisture free environment, such as an N₂environment. If a desiccant comprising a substantially moisture-freepolymer is utilized in an assembly process in a moisture-freeenvironment, this moisture absorption and subsequent post-encapsulationoutgassing can be avoided. In certain embodiments, a substantiallymoisture-free desiccant may be provided by heating the polymer to drivewater out of the polymer. In certain embodiments, the curing process maydrive some or all of the water out of the polymer, and may be modifiedas discussed above to provide additional heating and drive additionalwater from the polymer. In other embodiments, storage at roomtemperature in moisture free environment may be sufficient to drive offthe water. This storage may be done in the in-line system in themanufacturing process, and the increase in the assembly time may bebalanced out by performing this drying process with large batches ofdesiccants. Because it may take more than 2 hours to evacuate themoisture from the matrix in such an environment, this time may bereduced through the application of heating or exposure to vacuum, asnoted above.

In another embodiment, the package may be heated after encapsulation ofthe desiccant to drive retained water from the polymer within thedesiccant after encapsulation of the desiccant within a MEMS package.Because of the wide variety of components comprising a MEMS package, thetemperature to which the MEMS package may be heated after encapsulationmay be very limited. In addition to the MEMS device itself, which maycomprise heat-sensitive materials, the seal holding the MEMS packagetogether may be damaged if the MEMS package is exposed to a temperaturewhich is too high. Generally, a temperature of 150° C. may be suitablefor certain MEMS devices. In embodiments in which a thermal epoxy isused as a seal, the MEMS package may be heated to a higher temperature.In embodiments in which a UV epoxy is used, the MEMS package may belimited to the use of a lower temperature, such as about 110° C. Incertain embodiments, temperatures at or above 70° C. may be used, and inother embodiments, temperatures at or above 120° C. may be used.

By heating the MEMS package, an encapsulated desiccant which contains amoisture-containing polymer may be heated so as to drive water out ofthe polymer and into the calcium oxide within the desiccant. Thisprocess may accelerate the chemical absorption of the moisture by thecalcium oxide, such that the water within the polymer may be fullydriven into the calcium oxide prior to operation of the MEMS device. Inaddition, water retained within the MEMS device, the seal, the interiorsurfaces of the MEMS package, and the atmosphere within the package mayalso be driven into the calcium oxide. Furthermore, other materials,such as solids which may be outgassed by any of the package contents,may be driven into the calcium oxide to bind with the calcium oxide, andprevent possible interference of such solids on the operation of thepackaged MEMS device.

FIG. 11 illustrates such an exemplary assembly process 400. In step 402,a desiccant is applied to a backplate. The process moves to a step 404where the backplate is aligned with a substrate supporting a MEMSpackage. In step 406, the backplate is sealed to the MEMS substrate soas to encapsulate both the desiccant and the MEMS device. In step 408,the MEMS package is heated to a temperature which will not damage any ofthe components therein, for a duration sufficient to drive moisture fromthe polymer in the desiccant to the calcium oxide. In certainembodiments, step 408 is completed before the MEMS device encapsulatedwithin the package is operated. In an embodiment in which the MEMSdevice comprises a movable layer actuatable to contact another layer,the step is preferably completed before the movable layer is actuated tocontact the other layer. The possibility is thus minimized of moistureor other materials which may cause stiction being present when themovable layer contacts the other layer.

While the above detailed description has shown, described and pointedout novel features of the invention as applied to various embodiments,it will be understood that various omissions, substitutions, and changesin the form and details of the device or process illustrated may be madeby those skilled in the art without departing from the spirit of theinvention. As will be recognized, the present invention may be embodiedwithin a form that does not provide all of the features and benefits setforth herein, as some features may be used or practiced separately fromothers.

1. A method of encapsulating a MEMS device, the method comprising:providing a MEMS package, said MEMS package comprising a substratesupporting a MEMS device; a backplate sealed to said substrate to formthe MEMS package; and a desiccant within the MEMS package, the desiccantcomprising calcium oxide and a polymer binding; and heating the MEMSpackage to cause moisture within the package to be chemically absorbedby the calcium oxide.
 2. The method of claim 1, wherein heating the MEMSpackage is performed prior to operation of the MEMS device.
 3. Themethod of claim 1, wherein heating the MEMS package causes moisture tobe driven from the polymer.
 4. The method of claim 1, wherein the MEMSpackage is heated at a temperature higher than about 70° C.
 5. Themethod of claim 4, wherein the MEMS package is heated at a temperaturehigher than about 120° C.
 6. The method of claim 1, wherein the polymercomprises phenol.
 7. A method of encapsulating a MEMS device, the methodcomprising: providing a substrate supporting a MEMS device; providing abackplate; providing a desiccant, said desiccant comprising calciumoxide and polytetrafluoroethylene; baking said desiccant underconditions sufficient to cause outgassing of organics retained therein;and sealing said backplate to said substrate to form a MEMS packageencapsulating the MEMS device and the desiccant.
 8. The method of claim7, wherein baking said desiccant comprises baking said desiccant at atemperature higher than about 105° C.
 9. The method of claim 8, whereinbaking said desiccant comprises baking said desiccant at a temperaturehigher than about 120° C.
 10. A method of encapsulating a MEMS device,the method comprising: providing a substrate supporting a MEMS device;providing a backplate; providing a desiccant, said desiccant comprisingcalcium oxide and a polymer binding; baking said desiccant underconditions sufficient to cause outgassing of solvents retained therein;and sealing said backplate to said substrate to form a MEMS packageencapsulating the MEMS device and the desiccant.
 11. The method of claim10, wherein providing a desiccant comprises providing a desiccant inpaste form, and wherein baking said desiccant comprising baking saiddesiccant at a temperature higher than that required to solidify thedesiccant.
 12. The method of claim 10, wherein providing a desiccantcomprises providing a desiccant in paste form, and wherein baking saiddesiccant comprising baking said desiccant for a duration longer thanthat required to solidify the desiccant.
 13. The method of claim 10,wherein baking said desiccant comprises baking said desiccant at atemperature higher than about 220° C.
 14. The method of claim 13 whereinbaking said desiccant comprises baking said desiccant at a temperaturehigher than about 280° C.
 15. A method of encapsulating a MEMS device,comprising: providing a substrate supporting a MEMS device, the MEMSdevice comprising a movable layer, wherein said movable layer comprisesaluminum and neodymium; providing a backplate; providing a desiccant;and sealing the backplate to the substrate to form a MEMS packageencapsulating the MEMS device and the desiccant.
 16. A MEMS package,said MEMS package comprising: a substrate supporting a MEMS devicecomprising a movable layer, wherein said movable layer comprisesaluminum and neodymium; a backplate sealed to said substrate to form aMEMS package encapsulating said MEMS device; and a desiccant sealedwithin said MEMS package.
 17. The MEMS package of claim 16, wherein saidmovable layer comprises a first sublayer and a second sublayer, saidfirst sublayer comprising nickel, and said second sublayer comprisingaluminum and neodymium, wherein said second sublayer is located betweensaid first sublayer and said substrate.
 18. The MEMS package of claim16, wherein said desiccant comprises calcium oxide and a polymerbinding.
 19. The MEMS package of claim 18, wherein said polymer comprisephenol.
 20. The apparatus of claim 16, further comprising: a processorthat is configured to communicate with said MEMS device, said processorbeing configured to process image data; and a memory device that isconfigured to communicate with said processor.
 21. The apparatus ofclaim 20, further comprising a driver circuit configured to send atleast one signal to the MEMS device.
 22. The apparatus of claim 21,further comprising a controller configured to send at least a portion ofthe image data to the driver circuit.
 23. The apparatus of claim 20,further comprising an image source module configured to send said imagedata to said processor.
 24. The apparatus of claim 24, wherein the imagesource module comprises at least one of a receiver, transceiver, andtransmitter.
 25. The apparatus of claim 20, further comprising an inputdevice configured to receive input data and to communicate said inputdata to said processor.
 26. An MEMS device package, comprising: meansfor absorbing moisture; first means for supporting said absorbing means;second means for supporting a MEMS structure, wherein the MEMS structurecomprises means for inhibiting the formation of protrusions on a surfaceof the MEMS structure; and means for sealing said second supportingmeans to said first supporting means.
 27. The MEMS device package ofclaim 26, wherein the means for absorbing moisture comprises adesiccant.
 28. The MEMS device package of claim 26, wherein the firstmeans for supporting said absorbing means comprises a backplate, thesecond means for supporting a MEMS structure comprises a substrate, andthe means for sealing comprises a seal.
 29. The MEMS device package ofclaim 26, wherein the means for inhibiting the formation of protrusionson a surface of the MEMS structure comprises a layer comprisingneodymium.